1. Field of the Invention
The present invention relates to a diffraction grating device in which index modulations are formed along the longitudinal direction of an optical waveguide.
2. Related Background Art
A diffraction grating device has index modulations formed in the longitudinal direction of an optical waveguide (optical fiber or planar optical waveguide). Especially, a diffraction grating device in which Bragg type index modulations are formed can selectively reflect, out of guided light components, a light component having a specific wavelength that satisfies the Bragg condition, and is therefore used as an optical filter or the like in an optical communication system. Such a diffraction grating device is manufactured by the two-beam interference method or phase grating method. In such a diffraction grating device, normally, index modulations having a predetermined period are formed in one region along the longitudinal direction of an optical waveguide. The diffraction grating device has only one reflection wavelength that satisfies the Bragg condition in accordance with the index modulation period.
On the other hand, a diffraction grating device having a plurality of reflection wavelengths has been proposed. For example, a diffraction grating device described in, e.g., M. A. Muriel, et al., “Field Distributions Inside Fiber Gratings”, IEEE Journal of Quantum Electronics, Vol. 35, No. 4, pp. 548-558 (1999) has index modulations formed in the core region of an optical waveguide in each of a plurality of regions separated from each other along the longitudinal direction of the optical waveguide. The diffraction grating device has a number of reflection wavelengths corresponding to the arrangement period (to be referred to as a “sampling period” hereinafter) of the plurality of regions.